1. Technical Field
The present invention generally relates to a method of preparing water extractable silicon-containing biomedical devices such as silicon hydrogels, e.g., contact lenses, intraocular lenses, and other ophthalmic devices.
2. Description of the Related Art
Hydrogels represent a desirable class of materials for the manufacture of various biomedical devices, including contact lenses. A hydrogel is a hydrated cross-linked polymeric system that contains water in an equilibrium state. Hydrogel lenses offer desirable biocompatibility and comfort. A silicon hydrogel is a hydrogel material including a silicon-containing monomer, the silicon containing monomer imparting higher oxygen permeability to the resultant hydrogel copolymer.
In general, a hydrogel polymeric ophthalmic device can be obtained by direct molding of a polymerizable monomer mixture wherein the mixture is dissolved in a non-aqueous, displaceable diluent. In this method the polymerizable mixture is placed in a mold having the precise shape of the final desired hydrogel (i.e., water-swollen) lens and the monomer/diluent mixture is then subjected to conditions whereby the monomer(s) polymerize to produce a polymer/diluent mixture in the shape of the final desired hydrogel lens. The polymerization is generally carried out in a non-aqueous medium because water can interfere with the polymerization reaction and adversely affect the properties of the resulting polymer. At some point, after the polymerization is complete, the diluent is displaced with water to produce a hydrated lens whose final size and shape approximates the size and shape of the original molded polymer/solvent article. Such direct molding of hydrogel contact lenses is disclosed in, for example, U.S. Pat. Nos. 4,495,313, 4,680,336, 4,889,664 and 5,039,459.
In U.S. Pat. Nos. 4,495,313, 4,889,664 and 5,039,459, the displaceable diluents disclosed are water-displaceable boric acid esters of polyhydric alcohols. In U.S. Pat. No. 4,680,336, the displaceable diluents disclosed are water-displaceable organic compounds selected on the basis of their viscosity and their Hansen cohesion parameters relative to the cohesion parameters of the polymeric component of the hydrogel to be prepared. Examples of diluents disclosed in the '336 patent include the ester reaction product of a di- or tricarboxylic acid or anhydride with a C3 to C4 alkane diol or triol, a polyol selected from the group consisting of hexane-1,2,6 triol and diglycerol, and a mixture of glycerol with a polyhydroxyl compound having at least two hydroxyl groups and a Hansen cohesion parameter δh of less than 25.
However, at least dihydric alcohol boric acid esters are not effective at solubilizing silicon-containing monomers in a silicon hydrogel monomer mix. This lack of solubilizing ability (as displaceable diluents in a silicon-containing monomer mix) may result in a hazy or opaque lens. It is known that monohydric alcohols of indeterminate molecular weight can be used to adjust the viscosity of at least dihydric alcohol boric acid esters. Heretofore, there has been no recognition or appreciation that low molecular weight monohydric alcohol boric acid esters can be used as a water-displaceable diluent/solubilizer/solvent/viscosity modifier in a silicon-containing biomedical device forming monomer mix which is capable of forming a water extractable silicon-containing biomedical device after polymerization.
Accordingly, it would be desirable to provide an improved method for making water extractable silicon-containing biomedical devices such as silicon hydrogel contact lenses that is simple and cost effective.